The present invention relates to a method for manufacturing a ceramic honeycomb structure relating to formation of the outer wall of the ceramic honeycomb structure and a coating material used for the method.
There is used a honeycomb structure made of heat resistant ceramic as a substrate for loading a catalyst for purifying harmful substances such as nitrogen oxides (NOx) and carbon monoxide (CO) in exhaust gas from automobiles or a filter for trapping particulate matter in exhaust gas thereon. The ceramic honeycomb structure has small mechanical strength since partition walls are thin and has high porosity. For the purpose of compensating for the strength and inhibiting breakages, there is provided an outer wall by applying slurry containing a ceramic powder (hereinbelow referred to as a “coating material”) on the outer periphery of a honeycomb structure (cell structure) subjected to grinding to have a fixed diameter, followed by drying and firing (see, e.g., JP-A-5-269388 and JP-B-2604876).
When a coating material is applied on the outer periphery of a honeycomb structure and dried, a crack is caused due to a contraction difference between the surface and the inside of the coating material. The crack in the outer wall, leads to deterioration in strength of the ceramic honeycomb structure, and, when a catalyst is loaded on the outer wall, it causes leakage of a catalyst solution from the crack. Therefore, in a coating material-drying step, observation on crack generation or the like in the coating material is performed, and amendment operation (reapplication) is manually performed with spending 45 seconds for one.
In addition, since a crack is caused due to a contraction difference between the surface and the inside of the coating material when the coating material is applied and dried, there has been provided a coating material where the contraction difference is suppressed by the use of a ceramic powder having a tap bulk density of 1.3 g/cm3 or more to hardly cause a crack in the outer wall (see e.g., JP-A-2004-231506).
In addition, in JP-A-2004-231506, the average particle size of the ceramic powder is adjusted, and the rate of the powder component having a particle diameter of 44 μm or less is adjusted to 80 wt % or less. By forming a dense layer in the vicinity of the ceramic honeycomb structure of the coating layer, water movement to the structure is limited, and a difference in contraction of the coating layer is suppressed to inhibit crack generation. However, a particle site distribution of the ceramic powder is not adjusted, and, when heat-drying is performed after the application, a crack cannot be inhibited completely.
Heretofore, when natural drying which hardly causes a crack was performed at ordinary temperature (25° C., 50% RH), drying of coating material required 24 hours or more. Further, since it was necessary to secure a space for drying, it was necessary to finish in a short period of time by heat-drying. Reduction in drying time and abolition of the amendment operation are preferable for improvement of takt time.
In addition, a jet mill used for pulverizing a aggregate has a defect that adjustment of both the particle size and the particle size distribution is difficult because the jet mil tends to make the particle size distribution broader (wider) as the particle size is increased because variance in the particle size increases and to make the particle size distribution sharper (narrower) as the particle size is decreased because particle size is uniformalized.
For example, as a ceramic powder (coarse particle aggregate) having a relatively large average particle size, a ceramic powder having an average particle size of 31 μm is prepared by the use of a mass production pulverizer. In the same manner, a ceramic powder having an average particle size of 16 μm (fine particle aggregate), which has about half the size of the coarse particle aggregate, is prepared. From the comparison, a particle size distribution of the ceramic powder having an average particle size of 31 μm becomes broad. FIG. 3A is a schematic view showing a coating material using a ceramic powder (coarse particle aggregate 42) having a relatively large average particle size. Since the average particle size is large though the particle size distribution is broad, the amount of liquid components 43 such as water is large. Therefore, since drying contraction is large in the case that the liquid components 43 such as water evaporate depending on drying conditions, a crack is caused. Thus, it has been difficult to prepare a ceramic powder for coating material, which hardly causes a crack upon drying, by the use of a mass production pulverizer.